Long ‘sexy’ fins don’t thwart swimming

U. QUEENSLAND (AUS) — Looking flashy to attract mates could be a risky strategy, but long, flamboyant fins don’t seem to be a burden for male threadfin rainbowfish, report scientists.

The researchers tested the evolutionary theory assumption that only the best individuals are able to bear the energetic or survival costs associated with “sexy” features.

Developing and maintaining an outrageously elongated tail, heavy horns, or conspicuously-bright scales could take a lot of energy and increase the risk of being captured by a predator,” says Associate Professor Robbie Wilson of University of Queensland’s School of Biological Sciences.

“A male who can thrive in spite of his sexy features? Now that’s attractive,” he says.

Male threadfins, found in streams and lakes across the top end of Australia, develop extensive trailing fin streamers, presumably at great cost to their swimming abilities.

Swimming is an energetically expensive activity, and changes to a fish’s hydrodynamic profile—for example, via a greatly elongated fin—should increase the energy needed to swim and decrease overall swimming performance.

Wilson, researcher Andrew Trappett, and colleagues measured the metabolic rates and sprint swimming speeds of male fish with varying natural fin lengths, and then re-measured males after experimentally shortening their fins. The findings are published in Functional Ecology.

“We found what we expected: that female threadfins preferred males with longer ornaments, but we were surprised to find no evidence that longer fins were hydrodynamically costly to males.”

“Males swam just as fast after their fins were shortened as before, and fin size didn’t affect metabolic rates during swimming,” he says. “Although a threadfin’s ornaments didn’t affect its metabolic rate or swimming speeds, it doesn’t mean its sexy fins aren’t costly.”

To ensure that the study didn’t measure the wrong thing, the next research step involves looking at how ornament length relates to a male’s ability to survive in more-natural conditions, where escaping predation depends on more than just swimming fast.